1. Field of the Invention
This invention pertains to valves and especially to gate valves suitable for high pressure service. The invention is particularly directed to improvements in the seals and sealing arrangement for a through-conduit, rising stem high pressure gate valve.
2. Description Of The Prior Art
Gate valves have been commonly used for many years and have been employed in a variety of services such as in the oil and/or gas services of the petroleum industry. However, such valves have been designed for service involving pressures of only several thousand pounds per square inch. Today due to the depleted energy supply it has become necessary to explore and produce oil and gas under very high pressures of the range 30,000 psi (pounds per square inch) or more. Such high pressures have required greater sealing capacity and better safety features in such valves.
Prior art gate valves generally consist of a valve body having inlet and outlet conduits with a chamber therebetween. A gate having an opening therethrough is disposed in a gate seat mounted within the chamber and the gate is mounted on a stem. The chamber is closed by a bonnet affixed to the valve housing. The stem reciprocates within the bonnet and is attached to a reciprocating apparatus for moving the gate between a closed position in which flow is blocked through the conduits and an open position in which flow is allowed through the conduits and the gate opening. Such a valve is illustrated in U.S. Pat. Nos. 3,538,938 and 3,696,831.
A gasket seal is provided to seal between the bonnet and the valve housing. A ring gasket has been used as shown in U.S. Pat. No. 2,433,638; 2,842,336; and 3,696,831 or an o-ring seal as shown in U.S. Pat. Nos. 3,095,004 and 3,135,285. It is well known to use a standard metal joint gasket received in grooves at the flange faces of two wellhead members as shown in U.S. Pat. Nos. 2,830,665 and 3,007,719. Such gaskets may have a diamond shaped cross-section or a T-shaped cross-section as advertised by Gray Tool Company on pages 2217-2272 in the 1970-71 Composite Catalog published by World Oil. A resilient steel ring with o-rings for sealing between a valve housing and a bonnet of a gate valve is shown in U.S. Pat. No. 2,780,233.
It is well known to seal between the gate seat and the valve housing by using an o-ring as shown in U.S. Pat. Nos. 2,957,492 and 3,348,567. Further in automatic pressure sealed gate valves shown in U.S. Pat. Nos. 3,095,004; 3,538,938; and 3,696,831 fluent sealant rings are disposed around the gate seats to provide sealing between the valve housing and gate seats in response to line pressure on fluent sealant reservoirs in communication with the fluent sealant rings, and o-rings are installed on each side of the fluent sealant rings to isolate the fluent sealant in the fluent sealant rings from contamination by line fluids. This same principle is used for seals in wellheads as shown in U.S. Pat. No. 2,952,479. It is also known to locate an o-ring seal on the end of the gate seat as shown in U.S. Pat. No. 3,696,831. U.S. Pat. Nos. 3,057,630 and 3,758,072 teach combining an elastomeric O-ring and a polyfluoroethylene polymer such as the brand "Teflon" or "nylon" or other low-friction plastic ring to make a seal ring. Further U.S. Pat. No. 3,103,366 teaches a deformable metal ring, as for example of aluminum, with an inner deformable plastic ring of polytetrafluororoethylene whereby the metal ring can adapt itself to slightly varying recess clearances and prevent the plastic ring from extruding through the clearances under pressure.
Generally the stem is sealed with the bonnet by means of a seal such as an o-ring, compressed sealant, or chevron packing as are shown in U.S. Pat. Nos. 2,657,989; 2,780,233; 2,842,336; 2,957,492; 3,095,004; 3,181,552; and 3,696,831. Various valves such as are disclosed in U.S. Pat. Nos. 2,657,721; 2,780,233; 2,842,336; 3,348,567 show back seats which are actuated to provide a metal-to-metal seal between the stem and the bonnet and/or valve housing when it is necessary to replace the packing around the stem. U.S. Pat. No. 3,013,769 discloses two separate seat rings mounted on the stem one of which will sealingly engage the bonnet either in the open position or in the closed position of the valve. U.S. Pat. No. 3,538,938 illustrates upper and lower stem sections with annular packing between the upper stem section and bonnet and between the lower stem section and valve housing, and further discloses upper and lower enlargements mounted on the upper and lower stem sections respectively.
The sealing systems and arrangements of the prior art gate valves were designed for working pressures in the range of several thousand pounds psi and are not able to service working pressures in the range of 30,000 psi. The prior art valves do not provide back-up seals if the stem seals blow out under high pressure and the stem seals are elastomeric seals and not metal-to-metal seals. Such sealing systems may permit the leakage of poisonous hydrogen sulfide gas under high pressure. Further gate seat seals will extrude and roll out of the seal grooves under high pressures.